Bede

David, however, was the youngest. The Lord is to be understood as the youngest among his brothers for a twofold reason: in which he filled the number eight, that is, those to whom he entrusted the sacraments of the Gospel when they were perfected in the law, both in time and in humility. For when the fullness of time had come, God sent his Son, born of a woman, born under the law, to redeem those who were under the law (Galatians IV). …

Commentary on Samuel ↗